1. Field of the Invention
This invention relates to a method for producing a fibrous web, in particular a paper web, paperboard web or tissue web, with which a material flow of higher consistency and a material flow of lower consistency are divided among respectively shared sections over the machine width and the two material flows of each section then mixed in a variable ratio to each other and the respective mixed flow fed via a headbox as a headbox stream to a mesh section, whereby the mixing ratio between these two material flows of different consistency in the respective mixed flow acts as the set-point variable for control of the gsm (grams per square meter) substance of the fibrous web over the machine width (it is understood that the gsm of the fibrous web can be referred to as the gsm substance of the fibrous web).
2. Description of the Related Art
In a dilution headbox, two material flows of different consistency, referred to as thick material and thin material for example, are mixed in a ratio which is variable over the machine width. Usually mesh water is used as thin material. The mix passes into the mesh section via the headbox stream. The mixing ratio acts as the set-point variable for the sectional control of the gsm (grams per square meter) substance of the fibrous web over the machine width. Hence it is possible to establish a constant gsm substance in the transverse direction in spite of interference variables such as different shrinkage over the width in the drying section.
Using the composition of the thick and thin material it is also possible with the mixing ratio to vary the ratio of filler material to fibers in the suspension. Furthermore, a changed composition of the suspension stream leads to a change of fibrous material retention and filler material retention in the mesh section. Hence the control of the gsm substance also influences in general the fraction of filler material in the paper.
It would be desirable to have not only a constant gsm substance over the machine width but also a constant filler material fraction over the machine width. However, as only one set-point variable is available for these two controlled variables, the control solution used hitherto is unable in general to accomplish this.
The object of the present invention is to create a simple and cost-effective method, as well as an accordingly improved apparatus of the type initially referred to, with which the previously mentioned disadvantages are reduced or even eliminated. In particular an at least essentially constant filler material fraction of the fibrous web over the machine width is also to be assured in as simple and reliable a manner as possible.